This invention relates to variable speed constant frequency (VSCF) electric power systems and, more particularly, to such systems which provide both starter and generator functions.
In airborne electrical power generation systems, it is desirable to have a single system which provides both the starter and generator functions. The weight savings on an airplane can be substantial when a dedicated starter is eliminated. For this reason, electrical power systems which are capable of providing engine start functions can provide both cost and weight savings.
Variable speed constant frequency power generation systems are commonly used for aircraft applications. One type of VSCF system includes a variable speed generator which supplies DC power to a pair of DC link conductors. An inverter circuit receives DC power from the link conductors and produces a constant frequency AC output. The inherent simplicity and reliability of DC link variable speed constant frequency systems has been established and it is desirable to modify the existing designs to provide starter capability.
When a VSCF power system operates as an engine starter, the electronic power converter is reversed via power contactors so that it derives its input power from a utility power bus and delivers power to the generator which in turn operates as a motor. The electronic converter typically includes a three-phase rectifier and a DC to AC inverter. The rectifier receives power from the line and feeds it to the inverter. Thus, the power drawn from the utility bus is characteristic of a three-phase full-wave rectifier circuit. The currents for such a rectifier are known to be high in harmonic content, i.e., 20% of the fifth harmonic, 14% of the seventh, 9% of the eleventh, 8% of the thirteenth, and so on.
A filter of proper design could be added to the system which would reduce the harmonic currents to acceptable levels; however, such a filter will necessarily add substantial weight to the system because it must be designed around the lowest harmonic (20% of the fifth harmonic). For example, a 60/75 kVA system would require a filter weighing approximately 30 pounds, or about one-third of the total generator system weight. Another disadvantage of a large filter is the leading power factor which it presents to the system. If the aircraft is powered by a conventional constant speed generator (for example, a ground power cart), then this large leading power factor load will cause the generator to self-excite, go to ceiling voltage, and damage the electrical system. Such an approach is not practical.
It is therefore desirable to design a VSCF starter/generator system in which the size of the required starter filter can be reduced.